LATE OTTAWAN DUCTILE SHEARING AND GRANITOID EMPLACEMENT IN THE HUDSON HIGHLANDS, NY

Large-scale, high-grade ductile shearing, migmatization, and the emplacement of a chemically diverse suite of granitoid plutons characterize the late- to post-Ottawan (<1030 to 925 Ma) geologic history of the northernmost New Jersey Highlands and the Hudson Highlands of New York. The ductile shear zones are large (0.5 to 2 km wide, 2-10 km long), subvertical to vertical, and all kinematic indicators consistently show dextral strike-slip deformation. Timing of deformation is broadly constrained to an upper limit of »1010 Ma based on the SHRIMP U-Pb zircon ages on crosscut metaplutonic rocks and a rough lower limit of »925 Ma based on hornblende ⁴⁰Ar/³⁹Ar age obtained from an undeformed, crosscutting granite pegmatite dike. At least four discrete granitoid suites were emplaced associated with this shearing event that range from A- and S-type granites to calc-alkaline, I-type gabrro and diorite. These are, in order of inferred or radiometric age: (1) the Sterling Forest Granite Sheets (not yet dated), the Mount Eve Granite (1020±4 Ma; Drake et al., 1991), the Canada Hill Granite (1010±6 Ma; Aleinikoff and Grauch, 1990), and the Lake Tiorati Metadiorite (1008±4 Ma). The Sterling Forest Granite Sheets consist of tabular bodies (5-200 m thick) of metaluminous to slightly peraluminous leucogranite (SiO₂ »75%) with transitional I- and A-type trace element characteristics. The Mount Eve Granite consists of about thirty small stocks (0.2-5 km²) of metaluminous syenogranite to quartz monzonite with strong A-type chemical signatures (e.g., high Fe, Ba, Zr, Y, HFSE, REE). The Canada Hill Granite is a peraluminous biotite leucogranite with S-type chemical affinities (e.g., high Si, Al, K, low Fe, highly variable REE content) consisting of small stocks (0.1-1 km²) and sheets (~2-4 m thick) that intimately associated with surrounding migmatitic metapelitic gneisses. The Lake Tiorati Metadiorite consists of small stocks and sheets (max. 1.5 km²) of mafic rocks (47-51% SiO₂) with strong I-type, calc-alkaline, continental arc chemical signatures (e.g., low Ti, HFSE). Dextral transcurrent shearing and granitoid emplacement is interpreted here to have resulted from a tectonic escape mechanism due to late- to post-Ottawan adjustments within the newly amalgamated Rodinian supercontinent.